The officer was on routine patrol Thursday at the elevated Q-line station on Ocean Parkway when he heard a commotion from the street below. Gugliara rushed downstairs to Brighton Beach Avenue, a center for Eastern European immigrant communities, where he was told by witnesses that Tcherniak had assaulted a 78-year-old woman, knocking her to the ground, a law enforcement official said.

Gugliara was trying to arrest Tcherniak and was calling for assistance when Tcherniak pulled out a hunting knife and slashed the officer, according to investigators.

As Tcherniak tried to flee, Gugliara fired and struck him twice in the torso, killing him. Tcherniak died near Sea Breeze Avenue and West First Street and police said the knife was found a few inches away from his hand.